The invention relates to acceleration sensors, and in particular to a reaction mass used with a pendulous accelerometer.
Pendulous accelerometers, for example, vibrating beam accelerometers, capacitive accelerometers, capacitive rebalance accelerometers, and translational mass accelerometers comprise a reaction mass. Existing design and manufacturing techniques for these devices are described in U.S. Pat. Nos. 4,495,815 "Mass And Coil Arrangement For Use In An Accelerometer," 5,396,798 "Mechanical Resonance, Silicon Accelerometer," 4,766,768 "Accelerometer With Isolator For Common Mode Inputs," 5,228,341 "Capacitive Acceleration Detector Having Reduced Mass Portion," 5,350,189 "Capacitance Type Accelerometer For Air Bag System," 4,335,611 "Accelerometer," and 3,702,073 "Accelerometer" which are incorporated herein by reference.
All practical pendulous accelerometers to date function on the principle of Neuton's law that force equals mass times acceleration. In many accelerometer applications high performance and small size are desirable. One problem with the design of small, high performance pendulous accelerometer sensors involves obtaining adequate reaction mass in a small space. A second problem with the design of small, high performance pendulous accelerometer sensors involves providing adequate isolation from the mounting structure such that mounting strains do not affect accelerometer performance.
Typical accelerometer sensors include a pendulous reaction mass, often referred to as a proof mass, suspended from a stationary frame by, for example, a flexural suspension member or some other form of pivot mechanism. This pivot constrains the reaction mass to only one direction of motion; the reaction mass is free to move along this one direction of motion unless restrained to the null position. The pendulous reaction mass must be restrained under acceleration by an opposing force which may be the result of a position feedback circuit. Alternatively, the accelerometer may be an open-loop device in which the opposing force may be supplied a spring in the form of, for example, pivot stiffness.
In a typical accelerometer sensor mechanism the pendulous reaction mass is suspended on a flexural suspension member inside an external support frame. Isolation is typically provided by mounting the supporting frame itself inside an isolation feature supported from a final exterior frame which provides mounting both to sensor covers and to the accelerometer housing. The above features as practiced in a typical vibrating beam accelerometer sensor are shown in FIGS. 1 and 2. The large exterior frame system is static and adds no mass to the active reaction mass. Additionally, any external strain couples through the exterior frame system directly across the length of the sensor mechanism. The resulting large frame dimensions tend to maximize the effect of error drivers, for example, thermal expansion mismatch, placing additional burden on the isolator function.